1. Field of the Invention
The present invention relates to improved flame retardant insulation compositions for wire and cable products. The insulation compositions of the invention have improved abrasion resistance and are comprised of an ethylene-silane copolymer, high density polyethylene, halogenated flame retardant, silanol condensation catalyst and zinc mercaptobenzimidazole/aromatic secondary amine stabilizer combinations.
2. Description of the Prior Art
Fire resistant polyolefin compositions are widely used for wire and cable insulation. In addition to insulating and fire resistant properties, the compositions must also have and retain over their service life certain physical properties. One physical property which has become increasingly important in recent years, due to ever more stringent service requirements, is abrasion resistance. Even though currently available compositions may satisfy the requirements of SAE J1128 which utilizes a sandpaper abrasion test, these same compositions may fail to meet the abrasion requirements of ISO 6722, Section 9.2. This latter specification has been adopted by most European car manufacturers and utilizes a much more demanding needle abrasion test.
For example, flame retardant compositions useful as insulation for wire and cable applications are disclosed in U.S. Pat. No. 6,372,828. The compositions utilize an ethylene-alkyl acrylate copolymer base resin with brominated flame retardant, antimony trioxide, hydrated inorganic filler, chemical crosslinking agent and two-component stabilizer consisting of a zinc salt of a mercaptobenzimidazole and aromatic secondary amine. While the formulations meet the requirements of UL Standard 1581 and, more specifically, UL Subject 758, Style 3321, they do not met the stringent needle abrasion requirements of ISO 6722, Section 9.2.
Accordingly, there is a need for flame retardant insulation compositions having improved abrasion resistance. It would be particularly advantageous if moisture curable insulation compositions having improved abrasion resistance were available. Moisture crosslinkable insulation compositions for wire and cable are known. These compositions utilize silane copolymers obtained by grafting unsaturated alkoxysilanes to ethylene polymers as described in U.S. Pat. No. 3,646,155 or by copolymerizing ethylene with an unsaturated alkoxysilane as described in U.S. Pat. Nos. 3,225,018 and 3,392,156.
The use of benzimidazoles to stabilize polyolefin compositions, particularly polyethylene and polypropylene, is known. U.S. Pat. No. 3,218,276 discloses the use of alkyl benzimidazole to stabilize fiber-forming polyolefins. Polypropylene fiber-forming compositions containing 0.2 to 2.0 percent benzimidazole with other conventional additives are disclosed. U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working. Zinc mercaptobenzimidazole is specifically mentioned.
The use of combinations of hindered phenols with various zinc salts of mercapto compounds to provide stabilization of cured and crosslinked polyolefins utilized as insulation for electrical conductors is disclosed in U.S. Pat. Nos. 4,260,661, 4,693,937, 4,797,323 and 4,824,883. Combinations of IRGANOX 1010 with the zinc salt of 2-mercaptobenzimidazole (ZMB), the zinc salt of 2-mercaptotolylimidazole (ZMTI) and the zinc salt of 2-mercaptobenzothiazole (ZMBT) are disclosed. U.S. Pat. No. 4,459,380 discloses combining a sterically hindered phenol with a zinc salt of a mercaptoimidazole to stabilize crosslinkable curable ethylene-propylene rubber compositions. The reference provides for the inclusion of other conventional additives, such as Sb2O3, halogenated compounds, fillers, silanes and crosslinking agents in the formulations.
U.S. Pat. No. 5,196,462 shows the use of such stabilizer combinations to stabilize thermoplastic elastomers and indicates that other antioxidants, such as phenols, thiodipropionates and quinolines may also be present.
Rubber/silicone compositions containing a metal benzimidazole, an aromatic secondary amine, an organopolysiloxane oil and organic peroxide are disclosed in U.S. Pat. No. 4,808,643.